Some rheological properties of an interpolymer complex formed in the mixtures of aqueous solutions of poly(acrylic acid) and methyl cellulose are investigated. In a diluted state, the complex exhibits polyelectrolyte properties, and in the shear field, it flows like a pure poly(acrylic acid) solution. From the experimental data obtained from the mixture flow in a longitudinal field, an effective relaxation time was calculated, It turned out to be higher than that for the initial components. The deviation of the experimental values of the viscosity from the calculated additive ones allowed us to determine the critical molecular weight of the poly(acrylic acid) below which the complex is not formed: M-cr approximate to 6 x 10(4), which is one order of magnitude higher than for the mixtures of spatially complementary macromolecules. The increase of the total polymer concentration in the mixture first leads to an increase of the complex size. Further increase of the polymer concentration and transition to the semidiluted state decreases the complex size and also decreases its solubility in water.